adding draft mode

[tlienart]

cc @limarta (if you could try this and check it works fine for you, I'd be grateful for the feeback)

+++
draft = true
+++

# ABC

Some content that you want to see locally but not when deploying

if you call serve() locally, the page will appear. If you call optimize(...) (which would be the case with the github action), that page will not be generated and if there's a file at the output location, it'll be removed.

closes #1044

[limarta]

seems to work!